In BWR, it is an important problem to suppress the SCC of the materials constructing the core structures and pressure boundaries (stainless steel, nickel-base alloys) from the viewpoint of improving the plant operating rate. SCC takes place when the three factors (materials, stress, environment) fall on one another. Accordingly, SCC can be mitigated by mitigating at least one of the three factors.
When a plant is operated, the core cooling water is radioactively decomposed by the intense gamma and neutron rays emitted from the core. As its result, the structural materials constructing the in-core structures and pressure boundaries come to be exposed to the core cooling water containing oxygen and hydrogen peroxide (both are the products of radiolysis) in an amount of several hundreds ppb and having a high temperature (in this invention, a temperature of 100° C. or more is referred to as high temperature; and the outlet temperature of core is 288° C. at the time of normal power operation). FIG. 2 illustrates the relation between crack growth rate (hereinafter, referred to as “CGR”) and electrochemical corrosion potential (hereinafter, referred to as “ECP”). It is apparent from FIG. 2 that CGR decreases when ECP drops. FIG. 3 illustrates the results of measurement on the relation between the concentrations of oxygen and hydrogen peroxide and ECP of type 304 stainless steel (hereinafter, referred to as “304SS”) in high-temperature water. Both oxygen and hydrogen peroxide show a higher ECP at a higher concentration. Accordingly, for mitigating SCC of structural materials exposed to the cooling water of reactor, it is necessary to reduce ECP, or to lower the concentrations of oxygen and hydrogen peroxide present in the reactor water.
As a technique for solving this problem, the technique of adding hydrogen from the feed water system (hereinafter, referred to as “hydrogen injection”) can be referred to. Hydrogen injection is a technique of reacting the injected hydrogen with the oxygen and hydrogen peroxide formed by the radiolysis of water to return them to water, and thereby decreasing the concentrations of oxygen and hydrogen peroxide in the reactor water. If the hydrogen injection is carried out, however, radioactive nitrogen 16 (hereinafter, referred to as “N-16”) formed by the radio-activation of water becomes readily migrating together with steam, and this N-16 enhances the dose rate of turbine building. FIG. 4 illustrates the relation between the concentration of hydrogen in the fed water and effective oxygen concentration ((oxygen concentration)+0.5×(hydrogen peroxide concentration)) and the relation between the concentration of hydrogen in the feed water and the relative value of main steam line dose rate. It is apparent from FIG. 4 that an increase in hydrogen concentration in the feed water brings about a rise in the relative value of main steam line dose rate, though it causes a decrease in the effective oxygen concentration.
For solving this problem, a technique of making an element of the platinum group adhere to the surface of material and thereby accelerating the reaction between hydrogen and oxygen and hydrogen peroxide (for example, see: (1) JP Patent No. 2766422). By this technique, ECP can be decreased while suppressing the rise in the main steam line dose rate.